This work aims to (1) identify the most conductive conditions for the generation of greenhouses gases (GHGs) in groundwater (e.g., hydrogeological contexts and geochemical processes) and (2) evaluate the ... [more ▼]

This work aims to (1) identify the most conductive conditions for the generation of greenhouses gases (GHGs) in groundwater (e.g., hydrogeological contexts and geochemical processes) and (2) evaluate the indirect emissions of GHGs from groundwater at a regional scale in Wallonia (Belgium). To this end, nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2) concentrations and the stable isotopes of nitrate (NO3−) and sulphate were monitored in 12 aquifers of the Walloon Region (Belgium). The concentrations of GHGs range from 0.05 µg/L to 1631.2 µg/L for N2O, 0 µg/L to 17.1 µg/L for CH4, and 1769 to 100,514 ppm for the partial pressure of CO2 (pCO2). The highest average concentrations of N2O and pCO2 are found in a chalky aquifer. The coupled use of statistical techniques and stable isotopes is a useful approach to identify the geochemical conditions that control the occurrence of GHGs in the aquifers of the Walloon Region. The accumulation of N2O is most likely due to nitrification (high concentrations of dissolved oxygen and NO3− and null concentrations of ammonium) and, to a lesser extent, initial denitrification in a few sampling locations (medium concentrations of dissolved oxygen and NO3−). The oxic character found in groundwater is not prone to the accumulation of CH4 in Walloon aquifers. Nevertheless, groundwater is oversaturated with GHGs with respect to atmospheric equilibrium (especially for N2O and pCO2); the fluxes of N2O (0.32 kg N2O-N Ha-1 y-1) and CO2 (27 kg CO2 Ha-1 y-1) from groundwater are much lower than the direct emissions of N2O from agricultural soils and fossil-fuel-related CO2 emissions. Thus, indirect GHG emissions from the aquifers of the Walloon Region are likely to be a minor contributor to atmospheric GHG emissions, but their quantification would help to better constrain the nitrogen and carbon budgets. [less ▲]

We report a data-set of CO2, CH4, and N2O concentrations in the surface waters of the Meuse river network in Belgium, obtained during four surveys covering 50 stations (summer 2013 and late winter 2013 ... [more ▼]

We report a data-set of CO2, CH4, and N2O concentrations in the surface waters of the Meuse river network in Belgium, obtained during four surveys covering 50 stations (summer 2013 and late winter 2013, 2014 and 2015), from yearly cycles in four rivers of variable size and catchment land cover, and from 111 groundwater samples. Surface waters of the Meuse river network were over-saturated in CO2, CH4, N2O with respect to atmospheric equilibrium, acting as sources of these greenhouse gases to the atmosphere, although the dissolved gases also showed marked seasonal and spatial variations. Seasonal variations were related to changes in freshwater discharge following the hydrological cycle, with highest concentrations of CO2, CH4, N2O during low water owing to a longer water residence time and lower currents (i.e. lower gas transfer velocities), both contributing to the accumulation of gases in the water column, combined with higher temperatures favourable to microbial processes. Inter-annual differences of discharge also led to differences in CH4 and N2O that were higher in years with prolonged low water periods. Spatial variations were mostly due to differences in land cover over the catchments, with systems dominated by agriculture (croplands and pastures) having higher CO2, CH4, N2O levels than forested systems. This seemed to be related to higher levels of dissolved and particulate organic matter, as well as dissolved inorganic nitrogen in agriculture dominated systems compared to forested ones. Groundwater had very low CH4 concentrations in the shallow and unconfined aquifers (mostly fractured limestones) of the Meuse basin, hence, should not contribute significantly to the high CH4 levels in surface riverine waters. Owing to high dissolved concentrations, groundwater could potentially transfer important quantities of CO2 and N2O to surface waters of the Meuse basin, although this hypothesis remains to be tested. [less ▲]

This work reviews applications of stable isotope analysis to the studies of transport and transformation of N species in groundwater under agricultural areas. It summarizes evidence regarding factors ... [more ▼]

This work reviews applications of stable isotope analysis to the studies of transport and transformation of N species in groundwater under agricultural areas. It summarizes evidence regarding factors affecting the isotopic composition of NO3−, NH4+ and N2O in subsurface, and discusses the use of 11B, 18O, 13C, 34S, 87Sr/86Sr isotopes to support the analysis of δ15N values. The isotopic composition of NO3−, NH4+ and N2O varies depending on their sources and dynamics of N cycle processes. The reported δ15N-NO3− values for sources of NO3− are: soil organic N – +3‰–+8‰, mineral fertilizers – −8‰–+7‰; manure/household waste – +5‰ to +35‰. For NH4+ sources, the isotopic signature ranges are: organic matter – +2.4–+4.1‰, rainwater – −13.4–+2.3‰, mineral fertilizers –−7.4–+5.1‰, householdwaste –+5–+9‰; animalmanure–+8–+11‰. ForN2O, isotopic composition depends on isotopic signatures of substrate pools and reaction rates. δ15Nvalues of NO3− are influenced by fractionation effects occurring during denitrification (ɛ=5–40‰), nitrification (ɛ=5–35‰) and DNRA (ɛ not reported). The isotopic signature of NH4+ is also affected by nitrification and DNRA as well as mineralization (ɛ=1‰), sorption (ɛ=1–8‰), anammox (ɛ=4.3–7.4‰) and volatilization (ɛ=25‰). As for theN2O, production of N2O leads to its depletion in 15N, whereas consumption – to enrichment in 15N. The magnitude of fractionation effects occurring during the considered processes depends on temperature, pH, DO, C/NO3− ratio, size of the substrate pool, availability of electron donors, water content in subsoil, residence time, land use, hydrogeology. While previous studies have accumulated rich data on isotopic composition of NO3− in groundwater, evidence remains scarce in the cases of NH4+ and N2O. Further research is required to consider variability of δ15N-NH4+ and δ15N-N2O in groundwater across agricultural ecosystems. [less ▲]

We investigated anaerobic nitrogen (N) cycling in the water column of Lake Kivu, a deep meromictic tropical lake in East Africa. Data were collected at one station in the Northern Basin and one in the ... [more ▼]

We investigated anaerobic nitrogen (N) cycling in the water column of Lake Kivu, a deep meromictic tropical lake in East Africa. Data were collected at one station in the Northern Basin and one in the Southern Basin, during two sampling campaigns (June 2011—dry season, and February 2012—rainy season). Short-term incubations of sulfide-free water with 15N-labeled substrates revealed high potential denitrification and dissimilatory nitrate reduction to ammonium (DNRA) rates (up to 350 and 36 nmol N produced L−1 h−1, respectively), while anaerobic ammonium oxidation (anammox) was lower (up to 3.3 nmol N produced L−1 h−1). However, anammox rates were 15 nmol N produced L−1 h−1 when 15 NH4+ was added at depths where NH4+ concentrations were very low (< 1 μmol L−1). With the addition of 5 μmol L−1 of 15 NO3− and 10 μmol L−1 of H2S, denitrification and anammox were stimulated in the Northern Basin, while the increase of DNRA rates was less notable. In the Southern Basin, the addition of H2S decreased denitrification rates, probably because of competition with DNRA, which increased, while no effect was observed on anammox. This study puts into evidence the co-occurrence of denitrification, anammox and DNRA, for the first time in a great tropical lake, and underlines the spatial heterogeneity of these processes. Contrary to numerous reports in literature, we show that anammox can significantly occur in presence of H2S, suggesting that the contribution of anammox in the N cycle may be underestimated. [less ▲]

We tested three treatments to preserve Posidonia oceanica leaves for the analysis of dimethylsulfoniopropionate (DMSP) and dimethylsulfoxide (DMSO): oven dried at 60 °C for 24 h, frozen at −20 °C, and ... [more ▼]

We tested three treatments to preserve Posidonia oceanica leaves for the analysis of dimethylsulfoniopropionate (DMSP) and dimethylsulfoxide (DMSO): oven dried at 60 °C for 24 h, frozen at −20 °C, and frozen-in-ice and kept at −20 °C. The DMSP content was analyzed by proxy as dimethylsulfide (DMS) by gas chromatography after alkaline cleavage at room-temperature. The DMSP leaf content of P. oceanica in samples that were oven dried at 60 °C for 24 h, then stored at room temperature decreased by 87% over 80 days of storage and then remained stable for about 88 additional days compared to the control. The DMSO leaf content of P. oceanica in samples that were oven dried increased nine-fold after 198 days of storage following drying compared to the control. Both the DMSP and DMSO leaf content of P. oceanica remained stable for 198 days compared to the control with frozen and frozen-in-ice treatments, which we both recommend as adequate protocols to preserve P. oceanica tissues for DMS(P,O) analysis. The annual average DMSP leaf content of P. oceanica at 10 m in the Bay of Calvi (Corsica, France) was 205 ± 58 μmol g−1 (fresh weight) based on samples preserved frozen, two orders of magnitude higher than the value we previously reported based on samples that were oven dried. The newly determined DMSP leaf content allows ranking P. oceanica as the highest DMSP producer reported to date among marine and inter-tidal autotrophs. [less ▲]

Seagrass meadows can be assimilated to seascape matrixes encompassing a mosaic of natural and anthropogenic patches. Natural patches within the Mediterranean Posidonia oceanica meadows show a structural ... [more ▼]

Seagrass meadows can be assimilated to seascape matrixes encompassing a mosaic of natural and anthropogenic patches. Natural patches within the Mediterranean Posidonia oceanica meadows show a structural particularity which consist in a duality of their edge types. One edge is eroded by bottom currents, while the adjacent meadow colonizes the bare sediments. This study aims to study the dynamics of these two edges through the investigation of the biogeochemistry (pH, total alkalinity, dissolved inorganic carbon, CO2, CH4, N2O, H2S, dissolved inorganic nitrogen, PO4 3−) within vegetated and unvegetated sediments. These observations are compared with the adjacent meadow to have a better understanding of the colonization processes. Our results reveal that the P. oceanica matrix shows differences from the vegetated edges of sand patches, especially with regard to nutrient availability, which is generally more important at the colonized edge (dissolved inorganic nitrogen up to 65.39 μM in June). A clear disparity also occurs between the eroded and colonized edge with both a seasonal and bathymetrical variation of leaf biomass with higher disparities at 10 m in June (colonized edge 1415 gDW m−2; eroded edge 1133 gDW m−2). Themost important contrasts during this study were assessed in June, suggesting that the warm period of the year is more suitable for sampling to highlight disparate characteristics in temperate seagrass meadows. These findings put into light the potential importance of biogeochemical processes in the dynamics of natural patch edges. We hypothesize that they may influence the structural dynamics of P. oceanica seascapes. [less ▲]

Travertine deposition is a landscape-forming process, usually building a series of calcareous barriers differentiating the river flow into a series of cascades and ponds. The process of carbonate ... [more ▼]

Travertine deposition is a landscape-forming process, usually building a series of calcareous barriers differentiating the river flow into a series of cascades and ponds. The process of carbonate precipitation is a complex relationship between biogenic and abiotic causative agents, involving adapted microbial assemblages but also requiring high levels of carbonate saturation, spontaneous degassing of carbon dioxide and slightly alkaline pH. We have analysed calcareous crusts and water chemistry from four sampling sites along the Hoyoux River and its Triffoy tributary (Belgium) in winter, spring, summer and autumn 2014. Different surface textures of travertine deposits correlated with particular microenvironments and were influenced by the local water flow. In all microenvironments, we have identified the cyanobacterium Phormidium incrustatum (Nägeli) Gomont as the organism primarily responsible for carbonate precipitation and travertine fabric by combining morphological analysis with molecular sequencing (16S rRNA gene and ITS, the Internal Transcribed Spacer fragments), targeting both field populations and cultures to exclude opportunistic microorganisms responding favourably to culture conditions. Several closely related cyanobacterial strains were cultured; however, only one proved identical with the sequences obtained from the field population by direct PCR. This strain was the dominant primary producer in the calcareous deposits under study and in similar streams in Europe. The dominance of one organism that had a demonstrated association with carbonate precipitation presented a valuable opportunity to study its function in construction, preservation and fossilisation potential of ambient temperature travertine deposits. These relationships were examined using scanning electron microscopy and Raman microspectroscopy. [less ▲]

Greenhouse gases (GHGs) are an environmental problem because their concentrations in the atmosphere have continuously risen since the industrial revolution. They can be indirectly transferred to the ... [more ▼]

Greenhouse gases (GHGs) are an environmental problem because their concentrations in the atmosphere have continuously risen since the industrial revolution. They can be indirectly transferred to the atmosphere through groundwater discharge into surface water bodies such as rivers. However, their occurrence is poorly evaluated in groundwater. The aim of this work is to identify the hydrogeological contexts (e.g., chalk and limestone aquifers) and the most conductive conditions for the generation of GHGs in groundwater at a regional scale. To this end, carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) concentrations, major and minor elements and environmental isotopes were monitored in several groundwater bodies of the Walloon Region (Belgium) from September 2014 to June 2016. The concentrations of GHGs in groundwater ranged from 1769 to 100519 ppm for the partial pressure of CO2 and from 0 to 1064 nmol/L and 1 to 37062 nmol/L for CH4 and N2O respectively. Over- all, groundwater was supersaturated in GHGs with respect to atmospheric equilibrium, suggesting that groundwater contribute to the atmospheric GHGs budget. Prior inspection of the data suggested that N2O in groundwater can be produced by denitrification and nitrification. The most suitable conditions for the accumulation of N2O are promoted by intermediate dissolved oxygen concentrations (2.5-3 mg L−1) and the availability of nitrate (NO3 ). These observations will be compared with the isotopes of NO3 . CH4 was less detected and at lower concentration than N2O, suggesting that groundwater redox conditions are not reducing enough to promoted the production of CH4. The results will be presented and discussed in detail in the presentation. [less ▲]

Since the 1950's, anthropogenic activities have increased nutrient river loads to European coastal areas. Subsequent implementation of nutrient reduction policies have led to considerably reduction of ... [more ▼]

Since the 1950's, anthropogenic activities have increased nutrient river loads to European coastal areas. Subsequent implementation of nutrient reduction policies have led to considerably reduction of phosphorus (P) loads from the mid-1980's, while nitrogen (N) loads were maintained, inducing a P limitation of phytoplankton growth in many eutrophied coastal areas such as the Southern Bight f the North Sea (SBNS). When dissolved inorganic phosphorus (DIP) is limiting, most phytoplankton organisms are able to indirectly acquire P from dissolved organic P (DOP). We investigate the impact of DOP use on phytoplankton production and atmospheric fluxes of CO2 and dimethylsulfide (DMS) in the SBNS from 1951 to 2007 using an extended version of the RMIRO-BIOGAS model. This model includes a description of the ability of phytoplankton organisms to use DOP as a source of P. Results show that primary production can increase up to 30% due to DOP uptake under limiting DIP conditions. Consequently, simulated DMS emissions also increase proportionally while CO2 emissions to the atmosphere decrease, relative to the reference simulation without DOP uptake [less ▲]

This work reviews the concentrations, the dynamics and the emissions of nitrous oxide (N2O) in groundwater. N2O is an important greenhouse gas (GHG) and the primary stratospheric ozone depleting substance ... [more ▼]

This work reviews the concentrations, the dynamics and the emissions of nitrous oxide (N2O) in groundwater. N2O is an important greenhouse gas (GHG) and the primary stratospheric ozone depleting substance. The major anthropogenic source that contributes to N2O generation in aquifers is agriculture because the use of fertilizers has led to the widespread groundwater contamination by inorganic nitrogen (N) (mainly nitrate, NO3−). Once in the aquifer, this inorganic N is transported and affected by several geochemical processes that produce and consume N2O. An inventory of dissolved N2O concentrations is presented and the highest dissolved concentration is about 18.000 times higher than air-equilibrated water (up to 4004 μg N L-1). The accumulation of N2O in groundwater is mainly due to denitrification and to lesser extent to nitrification. Their occurrence depend on the geochemical (e.g., NO3−, dissolved oxygen, ammonium and dissolved organic carbon) as well as hydrogeological parameters (e.g., groundwater table fluctuations and aquifer permeability). The coupled understanding of both parameters is necessary to gain insight on the dynamics and the emissions of N2O in groundwater. Overall, groundwater indirect N2O emissions seem to be a minor component of N2O emissions to the atmosphere. Further research might be devoted to evaluate the groundwater contribution to the indirect emissions of N2O because this will help to better constraint the N2O global budget and, consequently, the N budget. [less ▲]